Fabrication of a nanoporous template from a diblock copolymer film - neutral brush

A perpendicular orientation of cylindrical microdomains in diblock copolymer thin films is achieved by control over polymer-surface interactions. The block which forms cylindrical microdomains is removed by UV exposure and a chemical rinse to yield a nanoporous polymer film. The porous film can be used as a template for electrodeposition of metal nanodots or as a mask for reactive ion etching.

Contributors:

T.P. Russell, Craig J. Hawker

Lab:

NSF Center for Hierarchical Manufacturing

Manufactured Material or Structure:

Polystyrene film with hexagonal array of nanopores

Chemical Composition:

Polystyrene

Physical Form:

Nanoporous thin film

Material Properties:

35 nm thick film containing a hexagonal array of 18 nm diameter pores with a period of 34 nm. Pores are hydrophobic.

Step 1:

Purchase or synthesize random copolymers P(S-r-MMA) with benzyl alcohol as an end group or P(S-r-BCB-r-MMA) with a molecular weight of 35,000 daltons and a PS/PMMA volume ratio of 0.58/0.42 or a PS/BCB/PMMA ratio of 56/2/42. These two random copolymers have been approved to have an equal interfacial interaction with PS and PMMA block.

Step 2:

Synthesize or purchase diblock copolymer P(S-b-MMA), consisting of PS and PMMA, with a molecular weight 88,000 daltons and a PS volume fraction of 0.72. This polymer will self-assemble into hexagonally packed array of PMMA cylinders.

Step 3:

Clean a silicon wafer with piranha solution to activate hydroxyl groups at the substrate surface. (This step is only required for using P(S-r-MMA) copolymers. For P(S-r-BCB-r-MMA), any kind of substrates, such as metal, metal oxide, semiconductor or polymer surfaces can be used with gentle cleaning.)

Step 4:

Spin coat a 1 wt% solution of P(S-r-MMA) or 0.3wt% solution of P(S-r-BCB-r-MMA) in toluene onto the fresh cleaned substrate to make a random copolymer film.

Step 5:

Anneal P(S-r-MMA) copolymer film at 170 °C under vacuum for 72 hrs or bake P(S-r-BCB-r-MMA) copolymer film at 250 °C under N2 for 20 mins to anchor the copolymers to the substrate.

Step 6:

Rinse off the excess, unanchored random copolymers with toluene, leaving a ~ 7 nm thick film, which behaves as a neutral layer to balance the surface interactions between PS and PMMA block.

Step 7:

Spin coat a 1 wt% solution of P(S-b-MMA) copolymers in toluene onto the modified substrate to make a block copolymer film with film thickness of 35 nm.

Step 8:

Anneal the film at 170 °C under vacuum for 48 hrs and then quench the sample to room temperature.

Step 9:

To cross-link PS and degrade PMMA, expose film to 254 nm ultraviolet light (25 J/cm2 dosage) for 35 mins. The sample should be under vacuum for this step to avoid ozone degradation.

Step 10:

Remove degraded PMMA by soaking polymer film in an acetic acid bath at room temperature for 20 minutes and then rinsing by deionized water.

Step 11:

The remaining structure will be a PS film with hexagonally ordered pores with 18 nm in diameter and 34 nm in spacing.

Process Notes:

This nanoporous thin film can be used as a scaffold or template for various applications. The nanoporous thick film can be made by an electric field aligning BCP process:Fabrication of a nanoporous template from a diblock copolymer film - electric field alignment

A long-range lateral ordered nanoporous thin film can be made by a solvent annealing process, which is another related self-alignment process:Fabrication of a nanoporous template from a diblock copolymer film - solvent annealing